In many applications it may be advantageous to have the ability to determine a position of a mobile device. Position location may be helpful for navigation, tracking, or orientation applications. The continual advancement of the performance of portable electronics, particularly the advancements in the performance of processors, allows position location capabilities to be added in a variety of devices.
For example, it may be desirable for an operator of a mobile telecommunications system such as a cellular telecommunications system to be able to determine the position of a mobile handset during communication with a base transceiver station (BTS) of the system. A system operator may desire position location capabilities, for example, to satisfy the U.S. Federal Communications Commission (FCC) E911 emergency position location mandate.
Mobile devices may implement one or more position location techniques depending on the position location signaling methods used in the position location system. For example, a mobile device may use time of arrival (TOA), time difference of arrival (TDOA), advanced forward link trilateration (AFLT) or some other position location technique. Examples of position location systems include those that are based on the Global Positioning System (GPS), those that augment the GPS system with terrestrial based beacons such as hybrid position location systems, and terrestrial based beacon position location systems. In one example, the mobile device can determine its position by determining absolute delay measurements to two terrestrial beacons or relative delay measurements to at least three terrestrial beacons.
Most terrestrial ranging systems incorporate a pseudo noise (PN) code in a direct sequence spread spectrum configuration. Each position location beacon can transmit a PN code that identifies the beacon. In a time division multiplexed forward link communication system, the position location beacons can be synchronized to transmit a corresponding position location PN code at substantially the same time.
A mobile device can identify a particular source, in part, by correlating a received PN spread signal with an internally generated version. However, a mobile position location device in a terrestrial based system can receive widely disparate signal powers. One problem that mobile devices encounter is associated with the receiving widely disparate signal powers.
In certain situations, the mobile device is only able to determine the position location signal from one beacon because the signal from the beacon is so strong that it interferes with signals from other beacons. The signal to noise ratio of the position location signals from the surrounding beacons is too low for the mobile device to extract useful ranging measurements. A mobile device may experience this effect when it is relatively near one beacon and relatively far from surrounding beacons. The effect is typically referred to as the near-far problem.
Therefore, it is desirable to have a position location signaling technique, system, and device that allow for high performance position location in a variety of conditions, including a near-far condition, and yet may be implemented in a practical manner.